Cathode ray tube device



3, 1943. G. HEPP ETAL 2,325,676

GATHODE RAY TUBE DEVICE Filed Feb. 2, 1942 INVENTOR ATTORNEY Patented Aug, 3, 1943 CATHODE RAY TUBE DEVICE Gerard Hepp and Jan van der Mark, Eindhoven,

Netherlands; vested in the Alien Property Custodian Application February 2, 1942, serial Nc.-429,226 In the Netherlands September 22, 1939 claims.

It has been found necessary and consequently it is usual to coat the neck of cathode-ray tubes at the inside with metal since otherwise it would be possible for the glass wall to take locally different potentials and the symmetry of the beam of rays might get lost (astigmatism). This metal coating usually extends till the conical part constituting the transition between the neck and the bulb and is mostly connected to the positive pole of a source of direct current and to earth. This coating is sometimes called the second anode of the tube (the rst anode is located closely behind thc cathode in front of the deflection means).

lf the tube is a so-called iconoscope, in other words, if it contains a light sensitive mosaic which is scanned Aby the cathode ray, the pictures obtained with the aid of this tube always exhibit more or less troublesome patches of light.

It has been found by applicant that these patches disappear at least partly if the said coating is not connected directly to the positive pole of the source of direct current, as it has been usual hitherto, but is set up through a condenser at a point of constant voltage.

The explanation for this phenomenon cannot be given with certainty, since the behaviour of electrons inside an iconoscope are n-ot yet known completely. It is very probable, however, that it is related to a more uniform distribution of the space charge with respect to the mosaic, brought about by the secondary electrons originating from this mosaic. When using the invention, the potential to which the latter will load the metal coating will be rendered .more equal to the potetial taken by the glass bulb-Wall than with the Way of circuiting hitherto usual.

Since now the coating cannot carry oi secondary electrons any longer", the full electronic ray will at last have to be carried off by the mosaic plate. Consequently, the variations of intensity of this ray will be reproduced to the extent of 100% in the output circuit of the iconoscope. This may result in over-loading of the amplifier following after the iconoscope, particularly during the periods in which the ray is completely suppressed, such as during flying back after line or picture scanning.

When using the invention, it may therefore be advantageous to collect part of the secondary electrons on a conductive screen which is arranged in the tube so that it will no longer be possible for the electrons to impinge the back of the mosaic plate.

The present invention includes also the iconoscope provided with such a screen.

The invention will be more clearly understood by reference to the accompanying drawing showing, by way of example, one embodiment thereof.

The iconoscope shown is of the known type and is constituted by a bulb i containing the mosaic screen 2 to be scanned, which bulb passes with a conical part 3 into a cylindrical tube d containing a cathode 5, a control electrode 6 and an optical system 1, 8, 9. Further, the tube d is surrounded by coils I and la serving for electromagnetic deection of the cathode ray. The interior of the tube wall at the point of the coils Il),A and II is covered with a metal coating l2 which extends up to and including the conical part 3. This coating is indicated by shading. The various supply wires are led into the tube by means'of a bush i3 which is fitted for this purpose with a certain number of contacts I4.

For the sake of more clearness of the drawing the supply wires are, however, shown as if each of them were led separately through the glass tube-wall.

The filament of the cathode 5 is supplied by a transformer I5" Whose secondary winding is connected through a movable contact I6 to a point of a potentiometer I1. This potentiometer is supplied with high voltage by a source of direct current which is not represented. The so-called first anode 9 is directly connected to the earthed positive end of the said potentiometer. Hitherto it has been usual to connect also the coating directly to this end.

According to the invention, however, a condenser I8 is interposed in this connecting line so that, as previously, rapid potential variations of this coating are prevented, it is true, but the vaverage potential is no longer fixed. This is now going to be determined by the space charges inside the bulb, such as is the case with the glass inner wall of this bulb.

Now the coating I2 thus blocked can no longer carry off secondary electrons originating from the screen 2 so that all of them have to return either to the front or to the back of that screen. This would mean that the current in the leading-out wire of the screen 2 is at any time equal to the full intensity of the current of the ray. Consequently, if the ray is periodically suppressed, for example at the end of the lines of scanning, very strong voltage impulses will impinge through an output impedancer 20 the amplier (not shown) following after the tube,

which amplifier consequently may be overcharged.

To avoid this drawback, the mosaic screen 2 is surrounded on all sides, except that on which is provided the light sensitive mosaic, by an auxiliary screen 2l which is insulated from the main screen 2 and i"s connected to earth through a leading-out wire 22 of its own. The secondary electrons originating from the space in front of the main screen 2 and which would otherwise be carried off via the side and back walls of this screen are now collected by the auxiliary screen 2l. The double function previously performed by the electrode I2 is thus divided over this coating and the auxiliary screen 2l.

A tube thus comprising an auxiliary screen is also advantageous in use when the coating I2 is directly connected to earth. In fact, in this case also part of the current of the ray will tend to impinge at last on the back of the screen 2, resulting in the above mentioned drawback. The auxiliary screen 2| reduces this part of the current of the ray to a minimum.

What we claim is: f

l. Apparatus for television transmission comprising an evacuated envelope, electron gun means within said envelope to develop an electron beam, a target electrode positioned and ,adapted to develop secondary electrons when scanned by said beam, an electrode exposed to` said target between said target and said electron gun electrically disconnected over a direct current conducting path with other structure within and without said envelope, and means enclosing the surface of said target electrode not directly exg posed to said beam to collect secondary elctrons liberated by said target and prevent such collection by the said surface of said target.

A2. Apparatus for television transmission comprising a tube having an electron gun to develop an electron beam, a photosensitive target adapted to be scanned and secondary electron emissive when scanned by said beam, a signal electrode coextensive and in capacitive relation with said target, an electrode between said gun and said target with no direct `electrical connection with said mosaic, said gun and said signal electrode, and a shield surrounding said signal electrode to collect secondary electrons and prevent collection of secondary electrons by said signal electrode.

3. Television transmitting apparatus comprising a tube having a secondary electron emissive photosensitive mosaic adapted to have developed thereon electrostatic charges representative of an optical image, a signal plate capacitively coupled to said mosaic, means to scan said mosaic with a beam of high velocity electrons to develop picture signals and inherently develop secondary electrons over the surface of said mosaic,

an electrode exposed to the surface of said mosaic incapable of collecting said secondary electrons with no direct current electrical connection to,

electrodes Within said tube, and a secondary electron collecting electrode completely surrounding said signal plate and extending slightly beyond and bordering the surface of said mosaic to collect said secondary electronsI and shield said signal plate from such collection.

4. Television transmitting apparatus comprising a tube enclosing an electron gun to develop an electron beam, a mosaic target having one side exposed to and adapted to be scanned by said beam, a signal plate on the opposite side of said mosaic, an electrode surrounding a portion of the path of said beam between said gun and said mosaic, and an electrically conductive shield surrounding said mosaic and enclosing said signal plate, means to connect said signal plate through a load impedance to a point of positive potential and means to connect said electrode through a non-conductive capacitive path to said point of positive potential.

5. Television transmitting apparatus comprising a tube having an electron source, an anode to develop an electron beam, a mosaic target facing and exposed to said source to be scanned by said beam, a capacitive signal plate adjacent the opposite side of said mosaic from that facing said sour-ce, a shield electrode enclosing i said signal plate and having a border portion extending in the direction of said source beyond the mosaic surface facing said source, and an electrode surrounding a portion of the beam path between said source and said mosaic target, a potential source" connected between said electron source and said signal plate to maintain said l signal plate positive with respect to said source 

